The invention relates to a catalyst on support for the selective oxidation of hydrogen sulfide to elemental sulfur, to a method for the preparation of such a catalyst, and to a method for the selective oxidation of oxidizable sulfur compounds, such as hydrogen sulfide, mercaptans and thiophenes, to elemental sulfur.
The necessity that gases which are further processed in chemical processes, are supplied to buyers or are discharged to the atmosphere, be purified of sulfur compounds, in particular hydrogen sulfide, mercaptans and thiophenes, is generally known. There are many natural gas occurrences where the natural gas occurs together with hydrogen sulfide. Also, large amounts of hydrogen sulfide are released upon the hydrogenating desulfurization of petroleum fractions, which are optionally further concentrated via an absorption-desorption method.
One of the most well-known methods for converting hydrogen sulfide to harmless elemental sulfur is the so-called Claus process. In this process, first about one-third of the H2S is combusted, whereby SO2 is formed. Owing to the combustion conditions, most organic contaminants in the gas stream are also combusted. The residual H2S reacts with the SO2 formed, first thermally and then in the presence of a catalyst to form water and elemental sulfur, according to the following reaction:2 H2S+SO2< - - - >2 H2O+3/n Sn  (1) 
In practice, the catalytic reaction is carried out in a number of catalyst beds arranged one after the other. In spite of this, however, the H2S is not converted quantitatively to elemental sulfur, mainly as a consequence of the position of the thermodynamic equilibrium of the reaction.
A residual content of H2S and SO2 remains. Now, generally it is not permitted to discharge H2S-containing residual gas, so that it must be combusted, whereby the hydrogen sulfide and other sulfur compounds, as is the elemental sulfur present in the gas phase, are oxidized to sulfur dioxide. Owing to the large amounts of H2S that are processed, the amounts of SO2 that are being emitted in this way are still considerable.
With environmental requirements becoming more stringent, this will no longer be allowed in view of the too high emission of sulfur dioxide involved. It is therefore necessary to further treat the residual gas from the Claus plant, the tail gas, in a tail gas plant.
It has been proposed to selectively oxidize the hydrogen sulfide that is present in the tail gas to elemental sulfur, optionally after hydrogenation of residual SO2, to H2S. For such selective oxidation processes specific catalysts are used.
U.S. Pat. No. 4,818,740, whose content is incorporated herein by reference, discloses such a catalyst, by the use of which side reactions are avoided to a large extent, while the main reaction occurs with a sufficient degree of conversion and selectivity.
The catalyst according to this patent contains a support of which the surface that can come into contact with the gas phase exhibits no alkaline properties under the reaction conditions, while on this surface a catalytically active material is provided. Further, the specific surface area of this catalyst is less than 20 m2/g and less than 10% of the total pore volume in this catalyst has a pore radius between 5 and 500 Å.
An improvement of the method disclosed in the above-mentioned U.S. Pat. No. 4,818,740 is described in European patent publication 409,353, whose content is incorporated herein by reference. This patent publication relates to a catalyst for the selective oxidation of sulfur-containing compounds to elemental sulfur, comprising at least one catalytically active material and optionally a support, which catalyst has a specific surface area of more than 20 m2/g and an average pore radius of at least 25 Å, while the catalyst exhibits substantially no activity for the reverse Claus reaction.
A third variant of such a catalyst is described in WO-A 95/07856. According to this patent publication the catalyst comprises at least one catalytically active material which has been applied to a support material, which support material, prior to the application of the catalytically active material, has been provided with at least one alkali metal promoter.
The effectiveness with regard to the conversion of H2S to elemental sulfur can generally be adversely affected by the occurrence of the following side reactions:
1. the subsequent oxidation of sulfur:1/n Sn+O2->SO2  (2) 
2. the reversed (or rather reversing) Claus equilibrium reaction:3/n Sn+2 H2O - - - >2 H2S+SO2  (3) 
Here the sulfur, once formed, enters into a reverse reaction with the water vapor also present, to form hydrogen sulfide and sulfur dioxide.
Tail gas generally contains, in addition to elemental sulfur, a considerable amount of water vapor, which amount can be between 10 and 40% by volume. This water vapor promotes the reversing Claus reaction to a great extent. Far-reaching removal of water vapor has evident technical disadvantages, such as the necessity for an additional cooling/heating stage, an additional sulfur recovery stage or a hydrogenation stage followed by a water-removing quench stage. A method whereby the conversion to elemental sulfur is hardly affected, if at all, by the water content of the feed gas is therefore desired.
Another important circumstance is that, generally, in the selective oxidation some excess of oxygen will be employed, not only to prevent “leakage” of H2S, but also for control engineering reasons. It is precisely this excess of oxygen, however, which can give rise to the subsequent oxidation of the elemental sulfur formed, so that the effectiveness of the process is adversely affected.
Depending on the choice of the catalyst and the reaction conditions, it is possible with such catalysts to obtain conversions of sulfur compounds which are fed to the Claus process, of up to about 99.2% by weight.
A drawback of the above-described catalysts for the selective oxidation of sulfur compounds is that upon substantially complete conversion of the hydrogen sulfide, the oxidation to SO2 of the sulfur formed increases with increasing temperature. An illustration thereof is given in the examples.
Technically, it is very difficult to accurately control the temperature at the end of the catalyst bed in the reactor. For processing large gas streams, as in the case of the tail gases from a Claus plant, in practice only adiabatic reactors are eligible. Since the selective oxidation reaction is exothermic, the inlet temperature and conversion determine the outlet temperature.
For achieving a sufficiently high conversion a minimum inlet temperature is required. On the other hand, it is endeavored to achieve as high a conversion as possible. With adiabatic reactors, this often results in a final temperature which is so high that in turn the selectivity decreases substantially, for instance to a value of about 80%. Clearly, there is a need for a catalyst which gives less rise to oxidation of sulfur to SO2, more particularly at higher temperatures.